One of methods for manufacturing a resin film extrudes a molten resin by using a die having a slit-shaped orifice (discharge port) referred to as a T-die. In particular, resin films for optical use are required to have high uniformity in film thickness and to be free of die lines (longitudinal streaks in an extrusion direction). Accordingly, it is required for a T-die used in this application as follows: the inner wall surface of a molten resin flow channel inside the T-die is smooth and has less friction with a molten resin; a lip portion at the tip end of an orifice has a high dimensional accuracy and has a sharp edge; and the T-die has high durability to maintain the aforementioned conditions for a long time. So far, in correspondence to the requirements, a coating layer such as a hard chromium plating layer has been provided to a molten resin flow channel and a harder hard coating layer has been provided at a lip portion.
Patent Document 1 describes to provide a WC-based coating layer onto a lip portion by means of flame spraying, the WC-based coating layer comprising an alloy formed by mixing WC particles as hard particles and Ni, Co, or Cr as a binder. A hard chromium plating layer is provided to the inner wall surface of a molten resin flow channel at portions other than the lip portion. However, since such a coating layer is relatively brittle, the layer tends to suffer from a defect such as peeling, cracking, or chipping upon finishing the edge portion by grinding and polishing after flame spraying. Such defect causes die lines to occur. Further, since adhesion between the WC-based coating layer and the hard chromium plating layer is not so satisfactory, peeling or cracking may possibly be caused between the two layers.
Patent Document 2 describes a T-die formed by bonding, to a main body, tabular lip member comprising a super hard alloy by means of a ceramic type adhesive. According to this constitution, a lip edge can be finished into a sharp edge. However, the super hard alloy has inferior adhesion with a hard chromium plating layer. Besides, it is difficult to apply plating finish to a portion other than the lip portion due to presence of the adhesion portion. Further, ensuring a sufficient adhesion strength requires a large adhesion area by enlarging the super hard alloy portion. However, this increases the material cost.
Patent Documents 3 and 4 each describe a method of forming a lip portion by bonding a corrosion resistant and wear resistant alloy powder through sintering and simultaneous diffusion bonding by Hot Isostatic Process (HIP) to a die main body comprising an austenite/ferrite double phase stainless steel alloy. A B (boron)-containing Ni-based alloy or cobalt-based alloy is used as the corrosion resistant and wear resistant alloy. A hard chromium plating layer is disposed to the inner wall surface of a molten resin flow channel at portions other than the lip portion. Since the lip portion obtained by the method described in the Patent Documents 3 and 4 has a dense metallographic structure with less defects, the edge portion can be formed into a sharp edge at high accuracy. However, an extremely complicated, expensive, and large-scaled manufacturing equipment is required for practicing the method described in the Patent Documents 3 and 4. Further, since the die main body in the HIP process is exposed to high temperature and high pressure, for example, at 1300° C. and 130 MPa, the die main body is distorted and bent. In this case, it is necessary to apply fabrication to the die main body including allowance therefor (particularly, refer to Patent Document 3). Thus, the method described in the Patent Documents 3 and 4 involves a problem of requiring enormous amount of labors and costs.